In the past, compounds have been selectively adsorbed from solutions by taking advantage of their affinity for a particular substrate. Generally, it has been recognized as an advantage to weakly adsorb a compound to a substrate so that the binding can be subsequently reversed.
It is often desired to selectively remove particular classes of synthetic industrial chemicals from manufacturing streams, as they are common components of wastewater, and can be observed as groundwater contaminants. Phenols represent one of the most important such classes of synthetic industrial chemicals and are often observed in effluents from various manufacturing or industrial operations, such as the manufacture of aniline, cumene-phenol processes, petroleum refining, petrochemical manufacture, coal gasification and coal liquification processes. Moreover, phenols are common components of pulp and paper wastes and have been observed as groundwater contaminants. However, despite the importance of treating phenol-containing wastewaters, current methods are not satisfactory and the search for improved methods continues.
Depending upon the process and the phenol concentration of the waste water, the aim is to recover the phenolic impurities and to reduce the level of phenols in the waste water to a negligible degree. In most cases, however, a recovery of the phenols is not practical since the expense of such recovery renders a process in which recovery is contemplated relatively impractical. Furthermore, the cost for the phenol recovery from solvents or adsorbents is often excessive.
Existing methods for removal of aromatics from aqueous solution typically comprise physical adsorption onto various media having a high surface activity, including active carbon, silica gel and synthetic-resin exchangers, metabolism by various microorganisms and chemical combustion in the presence of various oxidants. Alternatively, phenols can be removed by extraction or scrubbing with selective organic solvents or by steam distillation.
Unfortunately however, each of these methods has some or all of the following drawbacks: (a) time consuming, (b) expensive, (c) ineffective in dealing with pollutants at the levels encountered and/or in reducing pollutant levels to those desired, (d) exhibit a temperature sensitivity which can result in large seasonal variations, and (e) have a narrow specificity with respect to the classes of aromatic compounds amenable to treatment. Moreover, physical and chemical treatment methods become exceedingly expensive when a low effluent concentration must be achieved.
In the adsorption process, practical considerations have limited the adsorbent to active carbon. An adsorption process for the removal of phenols from waste waters, using active carbons, has various problems associated with it. For example, nonphenolic constituents of the wastewater are deposited on the active carbon which cannot be readily removed. Such materials include resinous and asphalt-type materials. In addition, the desorption or regeneration processes have a tendency to produce resinous and asphalt-type materials which are not completely released or destroyed by the active carbon so that the active carbon must often be replaced.
However, in many wastewater treatment applications, the activated carbon requirements have dramatically increased due to the need to meet more stringent effluent standards. Thus, although the activated carbon adsorption process is economically disadvantageous and the carbon particles have limited reusability, a treatment of the active carbon for reuse is practically mandated by the high cost of the adsorbent.
Chitosan, a polyglucosamine derived from chitin, has been shown to be effective as an adsorbent. Chitin can be obtained from sources such as crustacean shells, and other animal and fungal by-products, and can be converted to chitosan by deacetylation. Methods for obtaining chitosan are described in U.S. Pat. Nos. 4,282,351, 4,368,322 and 4,835,265.
Portier, in U.S. Pat. Nos. 4,882,066 and 4,775,650, describes compositions and processes for removal of metals contaminants or halogenated organic compounds from liquid streams polluted with these materials. The compositions are characterized as porous solids on the surfaces of which thin films of chitinous material are dispersed.
Microbiological dephenolation approaches have shown great promise for treating phenolic wastes (e.g., see Donaldson et al., Environ. Prog. 1987, vol. 6, pp. 205-211, and Worden and Donaldson, Biotechnol. Bioeng. 1987 vol. 30, pp. 398-412). For example, it is known to dephenolate waste waters by biological degradation of the phenolic impurities in synthetic biological environments and installations. However, for small volume wastes which are generated discontinuously, microbiological treatment has been plagued by instabilities resulting from the toxicity of these compounds to the microbial population (Jones et al., J. Gen. Microbiol. 1973 vol. 74, pp. 139-148; Yang and Humphrey, Biotechnol. Bioeng. 1975 vol. 17, pp. 1211-1235).
The idea of using enzymes for treating phenol-containing wastewaters was first proposed in the 1980s. Early workers suggested that enzymes could be used to convert soluble phenols and anilines present in wastewater into insoluble and apparently non-toxic polyphenolic precipitates which could be removed by filtration. Enzymes considered for this phenol precipitation included peroxidases (Klibanov and Morris, Enzyme Microb. Technol. 1981 vol. 3, pp. 119-122; Klibanov et al., J. Appl. Biochem. 1980 vol. 2, pp. 414-421, Science 1983 vol. 221, pp. 259-261; Aitken et al., Water Res. 1989 vol. 23, p. 443), laccases (Shuttleworth and Bollag, Enzyme Microb. Technol. 1986 vol. 8, pp. 171-177) and tyrosinases (Allow et al., Biotechnol. Bioeng. 1984) vol. 26, pp. 599-603. Advantageously, enzymes can react with a wide range of phenols even under dilute conditions and that these enzymes are likely to be less sensitive to operational upsets than microbial populations.
However, treatment of wastewater by the addition of peroxidase and hydrogen peroxide, although effective for its intended purpose, was found to have serious shortcomings which precluded it from being commercially attractive. In particular, hydrogen peroxide is expensive, unstable on storage and short lived in a real waste stream situation where metal salts, sunlight, and bacteria rapidly degrade the peroxides to oxygen and water. As a result, others have attempted to capture the advantages of enzymatic oxidation of the phenolic impurities in wastestreams, while overcoming the disadvantages.
For example, in U.S. Pat. No. 4,485,016, Hopkins teaches the removal of at least one organic compound, including aromatic hydroxy compounds, aromatic amines and phenols, from contaminated waste water. By this process the wastewater is treated with a treatment agent consisting essentially of peroxidase, an enzymatic agent consisting of alcohol oxidase and a straight chain C.sub.1 to C.sub.4 alcohol or glucose oxidase and glucose, and an azide salt, followed by separation of the precipitate by standard physical means, e.g., filtration, centrifugation, sedimentation, and the like.
In U.S. Pat. No. 4,623,465, Klibanov claims a process in which an oxidative enzyme, peroxidase, is added to the aqueous solution to be treated along with the enzyme's co-substrate, hydrogen peroxide. This mixture converts susceptible aromatic compounds to radicals which in a subsequent, presumably non-enzymatic, step couple with one another to eventually aggregate into a precipitate. Susceptible aromatics are hydroxy- and amino-substituted compounds with various other substituents such as alkyl, alkoxy, halo and fused aryl. In particular, Klibanov teaches the specific use of horseradish peroxidase for the oxidation of phenols to form a filterable precipitate.
Peroxidases are enzymes that catalyze chemical reactions that normally involve the transfer of hydrogen radicals from organic substances to substrates comprising peroxides. Such reactions may be complex, and may involve many different substances. Theoretically, the horseradish peroxidase reacts with phenol by removing a hydrogen radical (one proton with one electron) from the hydroxide group on the phenol. Thus, the phenol is thereby converted to an aromatic free radical, which participates in a subsequent reaction that depends upon other substances that are present in the solution, while the hydrogen radical reacts with hydrogen peroxide to form water.
The findings are also described in a series of publications and reports from Klibanov and co-workers, including: (1) Science 221:259-261 (1983); (2) Klibanov, National Technical Information Service, PB84-138155, 1-18 (1983); (3) Detoxification Hazard Waste (Symp., 1981), Edited by J. H. Exner, Ann Arbor Sci., CA 98(12):95130c (1982); (4) Atlow et al., Biotechnology and Bioengineering, XXVI:599-603 (1984); (5) Klibanov et al., Enzyme and Microbiol. Technol. 3(2): 119-22 (1981); (6) Klibanov, Enzyme Engineering 6, 3(2):319-325 (1982); and (7) Klibanov et al., Journal of Applied Biochemistry 2:414-421 (1980).
The specific improvement claimed by Klibanov is the ability of such a system to effect the clearance of compounds which are not substrates of the enzyme as long as there is a good substrate (a hydroxy- or amino-aromatic compound) also present. Thus, the clearance of a poor substrate is augmented in the presence of a good substrate. There are, however, aspects to this approach which lower its economic feasibility: (a) it requires large amounts of enzyme because the enzyme appears to be progressively inactivated under the reaction conditions; (b) relatively high concentrations of hydrogen peroxide are employed which are themselves inhibitory to the enzyme; (c) the treatment requires 3 to 24 hours for completion; (d) a final filtration or centrifugation step is required to remove the precipitate generated; and (e) each of the pure compounds studied appears to require removal at a different pH.
In U.S. Pat. No. 4,765,901, Field teaches a process for treating waste water by subjecting the contaminating phenolic compositions to oxidalive treatment, specifically by polyphenoloxidase enzymes, to increase the phenolic polymerization into polyphenolic aggregates for physical removal, e.g., precipitation, filtration or the like. The process is defined as oxidalive detoxification as opposed to oxidative dephenolization. Thus, the primary purpose of the process is a pretreatment to convert phenolic compounds into a harmless form, not a process of purification.
Field determined that waste water contaminated by phenolic impurities can be purified anaerobically in an excellent manner if, prior to the anaerobic purification, the waste water is subjected to an oxidative treatment. The oxidative treatment serves to increase the phenolic degree of polymerization, the formation of larger polymers from the smaller phenolic compounds originally present reducing or eliminating the methanogenic toxicity of the phenolic compounds originally present. Thus, Field teaches that the degree of polymerization, and consequently the toxicity to methanogenic bacteria, can therefore be influenced by an oxidative pretreatment.
In U.S. Pat. No. 5,051,184, Taylor claims the removal from aqueous solution of aromatic compounds, specifically including phenols, using an oxidative enzyme which is immobilized on a solid surface. Taylor teaches that the inhibitory action of elevated hydrogen peroxide concentration in batch processes is obviated by carrying out the clearance process at substoichiometric concentrations of peroxide in a flowing stream. The peroxide is continuously added to the mixture and collection and removal of the precipitate is accomplished by allowing the precipitate to accumulate on the filter on which the enzyme is immobilized.
However, there appear to be three rather serious problems involved in the methods for the enzymatic precipitation of phenols from wastewaters. These problems become apparent in light of the enzymatic and subsequent nonenzymatic reactions which occur in the presence of tyrosinase: ##STR1##
Quinones are rapidly formed from the tyrosinase-catalyzed oxidation of phenols. These quirtones are reactive and undergo nonenzymatic conversion to form additional, more stable intermediates. The more stable intermediates slowly undergo oligomerization reactions which ultimately can yield high molecular weight, insoluble polyphenolics.
The first problem with the enzymatic phenol precipitation approach is that the oligomerization and precipitation reactions are generally slow, requiring hours to days for completion. Secondly, unless initial phenol concentrations are very high, oligomerization may be limited to the formation of low molecular weight oligomers, which remain in solution rather than precipitating. Finally, the enzymes can be inactivated by the reactive intermediates generated from their reactions (Atlow et al., Biotechnol. Bioeng. 1984 vol. 26, pp. 599-603; Canovas et al., Biochem. Biophys. Acta 1987 vol. 912, pp. 417-423). Thus, despite the advantageous fact that the enzymes can react with a range of phenolics, even under dilute conditions, the previously-proposed approach of using enzymes for phenol precipitation has serious practical limitations.
Thus, there has been a long felt need in the art for a reliable, efficient and economical method for the selective separation of phenolic impurities from a wastestream which is not hampered by the above mentioned limitations. It is precisely this long-felt need which the present inventive two-step process for the selective separation of phenols from a wastestream was designed to meet.